Noise limiter circuit



April 1944- c. c. MARTINELLI I NOISE LIMITER CIRCUIT 0 Filed May 2'7, 1941 KL /0, 5004 0 A2. 0E2 MILL/9706 IF Iii/P107516 IICTl/ff 0077 07 Ennentor (0'0 6: MM

Gttorneg Patented Apr. 4, 1244 UNITED sTATssrATENT OFFiCE NOISE LIMITER CIRCUIT Ciro c. Martinelli, Oaklyn, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application May 27, 1941, Serial No. 395,303

I I 7 Claims. My invention relates to noise limiter circuits for radio receivers and is especially applicable to the sound channel of a television receiver.

An object of the invention is to provide an improved noise limiter circuit which is unusually- Karr patent, the limiting or gate voltage is one which varies in accordance with average carrier whereby manual adjustment of the gate voltage will not be necessary if the sound signal fades.

The present invention is an improvement on the above-described noise limiter circuit. According to my invention, the circuit is so designed that the gate voltage substantially follows the envelope of the sound-modulated carrier wave (1. e., follows the signal itself) whereby the noise-reducing ability of the circuit is greatly improved. The feature of having a gate or limiter voltage follow the said envelope is not new in itself, but it is new as applied to this type of circuit.

The invention will be better understood from v the following description taken in connection combined with the noise limiter.

Referring to Figure 1, the invention is shown applied to the sound channel of a television receiver which may comprise a first detector and oscillator indicated at In, an intermediate freget onto the plate of the limiter diode through quency amplifier It, and a second detector l2. 1 The pass band of the I. F. amplifier ll preferably is-several times the .band width of the sound signal. For example, the amplifier pass band may be 200,000 cycles wide, while the sound signal may have a frequency band lying between former l6, theplate of the diode being connected to the upper end of the transformer secondary and the cathode of the diode being connected to the lower end of the secondary through resistors I1 and I8.

The resistors l1 and I8 are bypassed by condensers l9 and 2|, respectively, these condensers having the usual capacity employed for bypassing the output resistor of a second detector.

The noise limiter circuit includes a diode 22 in series with a resistor R shunted by a condenser C. This series combination of diode and RC network is connected across the resistor I! through an A. C. connection including a condenser 23 and acrossthe resistor I8" through a D. C. connection including a resistor 20. I

It will be seen that the D. C. or average carrier voltage across resistor 18 is applied to the plate of diode 22 with positive polarity normally to maintain a flow of diode current through the resistor R from across which the sound output is taken. This average carrier voltage, however, is not by itself the gate or limiter voltage for the noise limiter. As will be explained below, the

portion of the gate voltage which follows the sound envelope is produced across the network RC. It may be noted that the noise pulses do not rent, thus producing a flow of sound signal current through the resistor R so long as the sound signal voltage doesnot exceed the gate or limiter voltage in amplitude.

If the signal voltage, a noise pulse for example, exceeds the gate voltage, the flow of diode current ceases and no signal appears across the output resistor R. I In accordance with my invention, the resistor R is shunted by a condenser C of such capacity that the voltage across the network RC follows the envelope of the sound-modulated carrier wave. when the gate voltage varies in this way,

as explained in the Holmes and Martinelli Patent No. 2,299,390, issued October 20, 1942, relating to a balanced type noise limiter, the noise limiter. action is unusually eflective, especially whenused in a signal channel having a pass ba'nds'everal times as wide as the frequency band of the signal being passed.

' farads.

The values of various circuit elements are indicated in Figure 1, merely by way of example, in

ohms, megohms, microfarads, and, micromicro- It will be apparent that the desired rapid variation of gate voltage cannot be obtained by omitting the condenser C and changing the time constant of the detector circuit portion Iii-2| as that would result in very poor detector action, giving a distorted'signal.

In general, it may be said that the RC network should have a time constant such that it has a cut-off just above the highest modulating frequency. Or, stated diflerently, the time conof a preemphasized signal, as described and claimed in Patent No. 22 99,391, of Ralph S. Holmes, for Radio receivers issued October 20, 1942'.

A suitable tone control circuit is illustrated in Figure 3, where parts corresponding to those in Figure 1 are indicated by the same reference characters. The RC network is shunted by a resistor 28 and a condenser 29 in series. A change in the value of resistor 28 changes the amount'of capacity effectively in shunt to the RC network, whereby its time constant may be changed to the desired value.

The circuit connections of my invention need not'be exactly as shown in Figure 1. For example, they may be as illustrated in Figure 2, where parts corresponding to those in Figure 1 are indicated by the same reference characters.

In Figure 2, the connection of the limiter diode 22 is reversed and the D. 0. connection is made across the resistor H rather than across the resistor l8. Also, here the cathode of the detector diode I3 is grounded rather than the junction point of resistors l1 and I8, a in Figure 1. If

desired, a single bypass condenser 3| may be connected across the resistors I! and IS. The operation of the circuit is the same as previously described.

I claim as my invention:

1. A radio receiver having a sound channel with a pass band which is several times the width of the frequency band occupied by the signal components passed by said channel, a detector sup.- pliedfrom said channel and having a load resistor across which the demodulated signal appears, a noise limiter of the series diode type connected across at least a portion of said load resistor, said limiter comprising a diode and an output resistor in series, and means for applying to said limitera gate voltage which varies substantially exactly with the useful-signal modulation envelope of the received carrier wave so that said varying gate voltage and said signal modulation envelope are substantially coincident.

2. A radioreceiver having a'sound channel with a pass band which is several times the width of the frequency band occupied by the signal components passed by said channel, a detector supplied from said channel and having a. load resistor across which the demodulated signal appears, a noise limiter of the series diode type connected across at least a portion of said load resistor, said limiter comprising a diode and an output resistor in series, means for applying .to said limiter a gate voltage which varies both with average carrier and with the useful signal modulation envelope of the received carrier wave.

3. In a radio receiver a detector for demodulating a modulated carrier wave, said detector including a load resistor across which the modulating signal appears and across which there is bypass capacity for bypassing the carrier frequency, a noise limiter circuit comprising a rectifler and a signal output resistor in series with each other to form a series combination, one end of said series combination being connected to an intermediate point on said load resistor, said output resistor having a certain amount of capacity thereacross, means for connecting the other end of said series combination to one end of said load resistor through a direct current connection to cause a direct current flow therethrough and to the other end'of said load resistor through an alternating current connection, the resistance value of said output resistor and thevalue of the capacity across said output resistor being such that the voltage thereacross substantially follows the useful signal modulation envelope of said carrier wave.

4. In a radio receiver, a detector for demodulating a modulated carrier wave, said detector including a diode and -a resistor across which the modulating. signal appears and across which ther is bypass capacity for bypassing the carrier frequency, a noise limiter circuit comprising a diode and a signal output resistor in series with each other to form a series combination, one end of said series combination being connected to an intermediate point on said load resistor, said output resistor having a certain amount of capacity thereacross, means for connecting the other end of said series combination to one end of said load resistor through a direct current connection to cause a, direct current flow there'- through and to the other end of said load resistor through an alternating current connection, the

resistance value of said output resistor and the value of the capacity across said output resistor being such that the voltage thereacross substantially follows the useful signal modulation en yelope of said carrier wave. 5. In a receiver for the reception of a modulated carrier wave, means for intercepting said wave and supplying it to a detector,' a detector for demodulating said carrier wave, said detector comprising a rectifier and a resistor in series with each other and inseries with said carrir wave supply means, said resistor having a certain amount of capacit thereacros to bypass signal at the carrier frequency, and a, noise lim- 'iter circuit comprising a rectifier and a signal end of said first resistor through an alternating current connection, the resistance value of said output resistor and the value of the capacity across said output resistor being such that the voltage thereacross substantially follows the useful signal modulation envelope of said carrier wave.

6. In a. receiver for the reception of a modu-- lated carrier wave, means for intercepting said accuse wave and supplying it to a detector, a detector for demodulating said carrier wave, said detector comprising a diode and a resistor in series with each other and in series with'said carrier wave supply means, and a noise limiter circuit comprising a diode and a signal output resistor in series with each other with one end or said series combination connected to a point on said first resistor, said output resistor having a certain amount of capacity thereacross, means for connecting the other end or said series combination to one end of said first resistor through a direct current connection to cause direct current flow therethrough and to the other end of said first resistor through an alternating current connection, the resistance value of said output resistor and the value of the capacity across said output resistor being such that the voltage thereacross substantially follows the useful signal modulation envelope of said carrier wave.

7. In a receiver having a sound amplifier channel, a detector supplied from said channel for demodulating a sound modulated carrier wave. said detector including a load resistor across which the modulating signal appears and across which there is bypass capacity for bypassing the carrier frequency, anoise limiter circuit comprising a rectifier and a signal output resistor in series with each other to form a series combination, one end of said series combination being connected to an intermediate point on said load resistor, said output resistor having a certain amount or capacity thereacross, means for connecting the other end of said series combination to one end of said load resistor through a direct current connection to cause a direct current flow therethrough and to the other end of said lead resistor through an alternating current connection, the resistance value 01' said output resistor and the value of the capacity across said output resistor being such that the voltage thereacross substantially follows the useful signal modulation envelope of said carrier wave and the pass band or said sound channel being several times as wide as the frequenc band of the sis= nal supplied to said detector.

C130 0. MARTINEILI. 

